Solar energy using photovoltaic effect requires active semiconducting materials to convert light into electricity. Currently, solar cells based on silicon are the dominating technology due to their high conversion efficiency. Recently, solar cells based on organic materials showed interesting features, especially on the potential of low cost in materials and processing. Judging from the recent success in organic light emitting diodes based on a reverse effect of photovoltaic effect, organic solar cells are very promising.
Organic photovoltaic cells have many potential advantages when compared to traditional silicon-based devices. Organic photovoltaic cells are light weight, economical in the materials used, and can be deposited on low cost substrates, such as flexible plastic foils. However, organic photovoltaic devices typically have relatively low quantum yield (the ratio of photons absorbed to carrier pairs generated. This is, in part, thought to be due to the second order nature of the intrinsic photoconductive process. That is, carrier generation requires exciton generation, diffusion and ionization. The diffusion length of an exciton is typically much less than the optical absorption length, requiring a tradeoff between using a thick, and therefore resistive, cell with multiple or highly folded interfaces, or a thin cell with a low optical absorption efficiency.
Conjugated polymers are polymers containing π-electron conjugated units along the main chain. They can be used as active layer materials for some types of photo-electric devices, such as polymer light emitting devices, polymer solar cells, polymer field effect transistors, etc. As polymer solar cell materials, conjugated polymers should possess some properties, such as high mobility, good harvest of sunlight, good processibility, and proper molecular energy level. Some conjugated polymers have proven to be good solar cell materials. Conjugated polymers are made of alternating single and double covalent bonds. The conjugated polymers have a 6-bond backbone of intersecting sp2 hybrid orbitals. The pz orbitals on the carbon atoms overlap with neighboring pz orbitals to provide π-bonds. The electrons that comprise the π-bonds are delocalized over the whole molecule. These polymers exhibit electronic properties similar to those seen in inorganic semiconductors. The semiconducting properties of the photovoltaic polymers are derived from their delocalized π bonds. The substituents of the polymers also largely influence the electronic properties. The optical bandgap, mobility and thin-film morphology are affected by both the type of functional group used as a substituent and the bulkiness and length of the side chain. Polymers which have only minor differences in the side chains will have large differences in the device performance.
There is a need in the art for polymer solar cells that exhibit increased solar conversion efficiency.